A Plague of Bogles by Catherine Jinks

Jem knows Alfred has retired from killing bogles, monsters that hide in dark places, but young children are still disappearing mysteriously in the city. Alfred agrees to use Jem as bait for the bogles, but something has changed. Bogles are popping up in the same area of the city, near an abandoned prison, but Alfred has never seen them living this close together. Fighting more than one bogle at a time is extremely dangerous, and the sewers offer many places to hide. To complicate matters further, Jem suspects Sarah Pickles, a woman who tried to kill him, is lurking around and may have something to do with the bogles. Jem wants revenge against her, but Alfred fears his anger may lead to his death.

This book is the second one in the series, and I recommend both of them. You should read the first book to fully understand what's going on. Birdie is no longer a main character in this book, as the story focuses on Jem. He wants to get off the streets, but he's afraid Alfred will kick him out or replace him with a Birdie. This insecurity causes Jem's character to make some poor decisions. The suspense kicks up every time Alfred and Jem hunt bogles, because the boy must let the monster almost grab him before Alfred slays it. Subplots are developed with Birdie and Sarah, and the author introduces the owner of a freak show to cause problems for Alfred. At the end of the book, Birdie says she's giving up bogling for good, but who knows? All in all, the author has created a very entertaining story.